IPV4 RAW AF INET TCP server client program with Select system call

IPv4 RAW_AF_INET TCP

• In this program, you are going to learn

• How to create a Socket ?

• How to write a data ?

• How to read a data ?

• How to use socket APIs ?

  • socket

  • write

  • read

Topics in this section,

• IPV4 RAW_AF_INET TCP SOCKET FAQs

• Step 1: Sequence Diagram for SERVER.c

• Step 2: Program for Server.c

• Step 3: Compile and Execute Server.c

• Step 4: Sequence Diagram for CLIENT.c

• Step 5: Program for Client.c

• Step 6: Compile and Execute Client.c

• Summary

IPV4 RAW_AF_INET TCP SOCKET : FAQs

Let us answer few basic questions in this socket

What does socket(AF_INET, SOCK_RAW, IPPROTO_TCP) do?

See Answer

This call creates a raw socket in the AF_INET address family for direct access to TCP packets.

When is it appropriate to use SOCK_RAW sockets with TCP?

See Answer

It’s suitable for tasks like packet capturing, network monitoring, or implementing custom protocols where direct access to TCP packets is needed.

Can this socket be used for regular TCP communication?

See Answer

While technically possible, it’s not recommended for regular communication due to increased complexity and potential security risks.

How does a raw TCP socket differ from a regular TCP socket?

See Answer

A raw TCP socket provides direct access to the TCP layer, allowing for manual packet manipulation, whereas regular TCP sockets handle packet details internally.

What are some use cases for raw TCP sockets?

See Answer

Use cases include network sniffing, packet analysis, security auditing, and developing custom network protocols.

How can I capture and analyze TCP packets using raw sockets?

See Answer

You can use the raw socket to capture TCP packets and analyze them using packet analysis tools like Wireshark.

Is error checking needed after creating the socket?

See Answer

Yes, checking for errors ensures that the socket is created successfully before proceeding with further operations.

Why is it important to check the return value of read() and write() in socket programming?

See Answer

It detects issues such as network errors or closed connections.

What is the purpose of the select system call in network programming?

See Answer

To block and wait for activity on one or more file descriptors.

How does select help in handling multiple sockets efficiently?

See Answer

It provides a way to wait for readiness on multiple sockets without blocking the entire program.

What types of file descriptors can be monitored using select?

See Answer

sockets, files, timerfd, socketpair, message_queue, Namedpipes and shared_memory.

What is the significance of the timeout parameter in the select function?

See Answer

It specifies the maximum duration to wait for any file descriptor to become ready.

How do you handle errors when using the select system call?

See Answer

Check the return value for -1 to detect errors, Use perror to print error messages.

How does select handle a set of file descriptors with different states (e.g., reading, writing, exception)?

See Answer

Preparing File Descriptor Sets:

select(readfds, writefds, exceptfds);

Setting Up Readiness Conditions:

If you are interested in monitoring file descriptors for readability, you add them to the readfds set.

FD_ZERO(&readfds);

FD_SET(fd1, &readfds);

Setting Up Writability Conditions:

If you are interested in monitoring file descriptors for writability, you add them to the writefds set.

FD_ZERO(&writefds);

FD_SET(fd2, &writefds);

Setting Up Exceptional Conditions:

If you are interested in monitoring file descriptors for exceptional conditions, you add them to the exceptfds set.

FD_ZERO(&exceptfds);

FD_SET(fd3, &exceptfds);

How does select Checking Ready File Descriptors?

See Answer

After select returns, you can check the sets to determine which file descriptors are ready for the specified conditions.

if (FD_ISSET(fd1, &readfds)) {
        // fd1 is ready for reading
}
if (FD_ISSET(fd3, &writefds)) {
        // fd2 is ready for writing
}
if (FD_ISSET(fd4, &exceptfds)) {
        // fd3 has an exceptional condition
}

What does it mean if select returns 0?

See Answer

No file descriptors are ready within the specified timeout.

Step 1: Sequence Diagram for SERVER.c

Step 2 : program for server.c

• There are many functions used in socket. We can classify those functions based on functionalities.

  • Create Socket

  • Bind Socket

  • Connect Socket

  • Select

  • Write data_packet

  • Read data_packet

  • Close socket

• socket() is used to create a new socket. For example,

sock_fd = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);

• bind() is used to associate the socket with a specific address and port. For example,

ret = bind(sock_fd, (struct sockaddr*)servaddr, sizeof(struct sockaddr_in));

• connect() is used in network programming to establish a connection from a client to a server. For example,

ret = connect(sock_fd, (struct sockaddr*)client_addr, sizeof(struct sockaddr_in));

• select() is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,

ret = select(sock_fd + 1, &read_fds, NULL, NULL, NULL);

• read system call in C is commonly used to read data from a file descriptor, such as a socket.

ret = read(sock_fd, recvbuffer, sizeof(recvbuffer));

• write system call in C is used to write data to a file descriptor, such as a socket.

ret = write(sock_fd, buffer, sizeof(buffer));

• close is used to close the socket To free up system resources associated with the socket. For example,

(void)close(sock_fd);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <errno.h>
#include <linux/ip.h>
#include <netinet/tcp.h>

#define PORT 50000
#define PORT_CLIENT 50001

struct sockaddr_in 
*servaddr = NULL,
*client_addr = NULL;
int sock_fd;

struct pseudo_iphdr {
   unsigned int source_ip_addr;
   unsigned int dest_ip_addr;
   unsigned char fixed;
   unsigned char protocol;
   unsigned short tcp_len;
};

unsigned short in_cksum (
uint16_t * addr, int len)
{
   int nleft = len;
   unsigned int sum = 0;
   unsigned short *w = addr;
   unsigned short answer = 0;

   while (nleft > 1) {
       sum += *w++;
       nleft -= 2;
   }

   if (nleft == 1) {
     *(unsigned char *) 
     (&answer) = 
    * (unsigned char *) w;
         sum += answer;
    }

    sum = (sum >> 16) + 
    (sum & 0xffff);
    sum += (sum >> 16); 
    answer = 
    (unsigned short) ~sum;
    return (answer);
}

void interrupt_handler (
int signum) {
    close(sock_fd);
    free(client_addr);
    exit(0);
}

void validate_convert_addr(
char *ip_str, 
struct sockaddr_in *sock_addr)
{
  if (ip_str == NULL) {
   perror("Invalid ip_str\n");
   exit(EXIT_FAILURE);
 }

 if (sock_addr == NULL) {
   perror("Invalid sock_addr\n");
   exit(EXIT_FAILURE);
 }

 printf("IP Address: %s\n", ip_str);

 if (inet_pton(AF_INET, ip_str, 
 &(sock_addr->sin_addr)) <= 0) {
    perror("Invalid address\n");
    exit(EXIT_FAILURE);
  }
}

int main (int argc, char *argv[])
{
  char buffer[1024] = 
  {0};
  unsigned char recvbuffer[1024] = 
  {0};
  int length, ret;
  char *string = 
  "Hello client";
  struct tcphdr *tcp_hdr = NULL;
  char *string_data = NULL;
  char *recv_string_data = NULL;
  char *csum_buffer = NULL;
  struct pseudo_iphdr 
  csum_hdr;

  signal (SIGINT, 
  interrupt_handler);
  signal (SIGTERM, 
  interrupt_handler);

  if (argc != 2) {
   printf("%s<ip-addr>\n",
   argv[0]);
   exit(EXIT_FAILURE);
  }

  sock_fd = socket(AF_INET, 
            SOCK_RAW, 
            IPPROTO_TCP);

  if(0 > sock_fd) {
     printf("unable to create\n");
     exit(0);
  }

  servaddr = (struct sockaddr_in *)malloc(
  sizeof(struct sockaddr_in));
        
  if (servaddr == NULL) {
     printf("could not allocate memory\n");
     goto end;
  }

  servaddr->sin_family = AF_INET;
  servaddr->sin_port = PORT;
  validate_convert_addr(argv[1], 
  servaddr);

  ret = bind(sock_fd, 
  (struct sockaddr *)servaddr, 
  sizeof(struct sockaddr_in));

  if (ret < 0) {
    printf("bind\n");
    goto end1;
  }

  client_addr = (struct sockaddr_in *)malloc(
  sizeof(struct sockaddr_in));
  
  if (client_addr == NULL) {
    printf("allocation memory\n");
    goto end2;
  }

  client_addr->sin_family = AF_INET;
  client_addr->sin_port =
  PORT_CLIENT;
  validate_convert_addr(argv[1],
  client_addr);

  ret = connect(sock_fd, 
  (struct sockaddr *)client_addr, 
  sizeof(struct sockaddr_in));
        
  if (ret != 0) {
     printf("error %d", errno);
     printf("connect returned error\n");
     goto end2;
  }
      
  string_data = (char *) 
  (buffer + sizeof(struct tcphdr));
  strncpy(string_data, string, 
  strlen(string));

  tcp_hdr = (struct tcphdr *)buffer;
  tcp_hdr->source = htons(PORT);
  tcp_hdr->dest = 
  htons(PORT_CLIENT);
  tcp_hdr->ack_seq = 0x0; 
  tcp_hdr->doff = 5; 
  tcp_hdr->syn = 1; 
  tcp_hdr->window = htons(200); 

  csum_buffer = (char *)calloc((
  sizeof(struct pseudo_iphdr) + 
  sizeof(struct tcphdr) + 
  strlen(string_data)), 
  sizeof(char));

  if (csum_buffer == NULL) {
     printf("allocate csum buffer\n");
     goto end1;
  }

  csum_hdr.source_ip_addr = 
  inet_addr("192.168.1.11");
  csum_hdr.dest_ip_addr = 
  inet_addr("192.168.1.14");
  csum_hdr.fixed = 0;
  csum_hdr.protocol = 
  IPPROTO_TCP;
  csum_hdr.tcp_len = 
  htons(sizeof(struct tcphdr) + 
  strlen(string_data) + 1);

  memcpy(csum_buffer, (char *)&csum_hdr, 
  sizeof(struct pseudo_iphdr));
  memcpy(csum_buffer + 
  sizeof(struct pseudo_iphdr), buffer, 
  (sizeof(struct tcphdr) + 
  strlen(string_data) + 1));

  tcp_hdr->check = (in_cksum(
  (unsigned short *) csum_buffer,
  (sizeof(struct pseudo_iphdr)+ 
  sizeof(struct tcphdr) + strlen(string_data) + 1)));

  printf("checksum is %x", tcp_hdr->check);
  free (csum_buffer);
  
  fd_set read_fds;

  while (1) {
   FD_ZERO(&read_fds);
   FD_SET(sock_fd, &read_fds);
 
   ret = select(sock_fd + 1, 
   &read_fds, NULL, NULL, NULL);
               
   if (ret == -1) {
       perror("select\n");
       break;
   }

   if(FD_ISSET(sock_fd, 
   &read_fds)) {
      memset(recvbuffer, 0, 
      sizeof(recvbuffer));

      ret = read(sock_fd, recvbuffer, 
      sizeof(recvbuffer));

      if (ret == -1) {
        perror("read error");
        break;
      }

      tcp_hdr = (struct tcphdr *)
      (recvbuffer + sizeof (struct iphdr));
      recv_string_data = (char *) 
      (recvbuffer + sizeof (struct iphdr) + 
      sizeof (struct tcphdr));
      
      if (PORT == ntohs(tcp_hdr->dest)) {
        printf("Received : %s\n", recv_string_data);
      }

      ret = write(sock_fd, buffer, 
      sizeof(buffer));
      
      if (ret == -1) {
        perror("write error");
        break;
      }
    }
   }

end2:
        free(client_addr);
end1:
        free(servaddr);
end:
        (void)close(sock_fd);

        return 0;
}

Step 3: Compile and Execute server.c

$ gcc -o server server.c

$ sudo ./server 127.0.0.1

IP Address: 127.0.0.1
IP Address: 127.0.0.1
checksum is c945
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
Received : Hello server
^C

Step 4 : Sequence Diagram for CLIENT.c

Step 5: program for client.c

• There are many functions used in socket. We can classify those functions based on functionalities.

  • Create Socket

  • Bind Socket

  • Connect Socket

  • Select

  • Write data_packet

  • Read data_packet

  • Close socket

• socket is used to create a new socket. For example,

sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);

• bind() is used to associate the socket with a specific address and port. For example,

ret = bind(sockfd, (struct sockaddr*)clientaddr, sizeof(struct sockaddr_in));

• connect() is used in network programming to establish a connection from a client to a server. For example,

ret = connect(sockfd, (struct sockaddr*)serveraddr, sizeof(struct sockaddr_in));

• select is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,

ret = select(sockfd + 1, &read_fds, NULL, NULL, NULL);

• read system call in C is commonly used to read data from a file descriptor, such as a socket.

ret = read(sockfd, recvbuffer, sizeof(recvbuffer));

• write system call in C is used to write data to a file descriptor, such as a socket.

ret = write(sockfd, buffer, sizeof(buffer));

• close is used to close the socket To free up system resources associated with the socket. For example,

(void)close(sockfd);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <errno.h>
#include <netinet/tcp.h>
#include <linux/ip.h>

#define PORT 50001
#define SERVER_PORT 50000

struct sockaddr_in 
*serveraddr = NULL, 
*clientaddr;
int sockfd;

struct pseudo_iphdr {
   unsigned int source_ip_addr;
   unsigned int dest_ip_addr;
   unsigned char fixed;
   unsigned char protocol;
   unsigned short tcp_len;
};

unsigned short in_cksum (
uint16_t * addr, int len)
{
   int nleft = len;
   unsigned int sum = 0;
   unsigned short *w = addr;
   unsigned short answer = 0;

   while (nleft > 1) {
       sum += *w++;
       nleft -= 2;
   }

   if (nleft == 1) {
      *(unsigned char *) (&answer) = 
      * (unsigned char *) w;
      sum += answer;
   }

   sum = (sum >> 16) + 
   (sum & 0xffff); 
   sum += (sum >> 16); 
   answer = (unsigned short) ~sum;
   return (answer);
}

void interrupt_handler (
int signum) 
{
  close(sockfd);
  free(clientaddr);
  exit(0);
}

void validate_convert_addr(
char *ip_str, 
struct sockaddr_in *sock_addr)
{
  if (ip_str == NULL) {
   perror("Invalid ip_str\n");
   exit(EXIT_FAILURE);
 }

 if (sock_addr == NULL) {
   perror("Invalid sock_addr\n");
   exit(EXIT_FAILURE);
 }

 printf("IP Address: %s\n", ip_str);

 if (inet_pton(AF_INET, ip_str, 
 &(sock_addr->sin_addr)) <= 0) {
    perror("Invalid address\n");
    exit(EXIT_FAILURE);
  }
}

int main (int argc, char *argv[])
{
   char buffer[1024] = 
   {0};
   unsigned char recvbuffer[1024] = 
   {0};
   int length, ret;
   char *string = 
   "Hello server";
   struct tcphdr *tcp_hdr = NULL;
   char *string_data = NULL;
   char *recv_string_data = NULL;
   char *csum_buffer = NULL;
   struct pseudo_iphdr csum_hdr;

   signal (SIGINT, 
   interrupt_handler);
   signal (SIGTERM, 
   interrupt_handler);

   if (argc != 2) {
     printf("%s<ip-addr>\n",
     argv[0]);
     exit(EXIT_FAILURE);
   }

   sockfd = socket(AF_INET, 
            SOCK_RAW, 
            IPPROTO_TCP);
      
   if(0 > sockfd) {
     printf("create socket\n");
     exit(0);
   }

   clientaddr = (struct sockaddr_in *)malloc(
   sizeof(struct sockaddr_in));
      
   if (clientaddr == NULL) {
      printf("allocate memory\n");
      goto end;
   }

   clientaddr->sin_family = AF_INET;
   clientaddr->sin_port = PORT;
   validate_convert_addr(argv[1],
   clientaddr);

   ret = bind(sockfd, 
   (struct sockaddr *)clientaddr, 
   sizeof(struct sockaddr_in));

   if (ret < 0) {
      printf(" bind\n");
      goto end1;
   }

   serveraddr = (struct sockaddr_in *)malloc(
   sizeof(struct sockaddr_in));
      
   if (serveraddr == NULL) {
      printf("allocate memory\n");
      goto end2;
   }
   
   serveraddr->sin_family = AF_INET;
   serveraddr->sin_port = SERVER_PORT;
   validate_convert_addr(argv[1],
   serveraddr);

   ret = connect(sockfd, 
   (struct sockaddr *)serveraddr, 
   sizeof(struct sockaddr_in));
      
   if (ret != 0) {
      printf("error %d", errno);
      printf("connect returned error\n");
      goto end2;
   }

   string_data = (char *) 
   (buffer + sizeof(struct tcphdr));
   strncpy(string_data, string, 
   strlen(string));

   tcp_hdr = (struct tcphdr *)buffer;
   tcp_hdr->source = htons(PORT);
   tcp_hdr->dest = htons(SERVER_PORT);
   tcp_hdr->ack_seq = 0x0;
   tcp_hdr->doff = 5; 
   tcp_hdr->syn = 1;
   tcp_hdr->window = htons(200);

   csum_buffer = (char *)calloc(
   (sizeof(struct pseudo_iphdr) + 
   sizeof(struct tcphdr) + 
   strlen(string_data)), sizeof(char));
      
   if (csum_buffer == NULL) {
      printf("allocate csum buffer\n");
      goto end1;
   }

   csum_hdr.source_ip_addr = 
   inet_addr("192.168.1.14");
   csum_hdr.dest_ip_addr = 
   inet_addr("192.168.1.11");
   csum_hdr.fixed = 0;
   csum_hdr.protocol = 
   IPPROTO_TCP;
   csum_hdr.tcp_len = 
   htons(sizeof(struct tcphdr) + 
   strlen(string_data) + 1);

   memcpy(csum_buffer, 
   (char *)&csum_hdr, 
   sizeof(struct pseudo_iphdr));
   memcpy(csum_buffer + 
   sizeof(struct pseudo_iphdr), 
   buffer, (sizeof(struct tcphdr) + 
   strlen(string_data) + 1));

   tcp_hdr->check = (in_cksum(
   (unsigned short *) csum_buffer,
   (sizeof(struct pseudo_iphdr)+ 
   sizeof(struct tcphdr) + 
   strlen(string_data) + 1)));

   printf("checksum is %x", 
   tcp_hdr->check);
      
   free (csum_buffer);

   fd_set read_fds;
      
   while (1) {

     ret = write(sockfd, 
     buffer, sizeof(buffer));
           
     if (ret < 0) {
        perror("read error");
        break;
     }

     FD_ZERO(&read_fds);
     FD_SET(sockfd, &read_fds);

     ret = select(sockfd + 1, 
     &read_fds, NULL, NULL, NULL);
           
     if (ret < 0) {
        perror("select\n");
        break;
     }

     if(FD_ISSET(sockfd, 
     &read_fds)) {
       memset(recvbuffer, 0, 
       sizeof(recvbuffer));
                
       ret = read(sockfd, 
       recvbuffer, sizeof(recvbuffer));
                
       if (ret < 0) {
          perror("read error");
          break;
       }

       tcp_hdr = (struct tcphdr *)
       (recvbuffer + sizeof (struct iphdr));
       recv_string_data = (char *) 
       (recvbuffer + sizeof (struct iphdr) + 
       sizeof (struct tcphdr));
                
       if (PORT == ntohs(tcp_hdr->dest)) {
           printf("Received : %s\n", 
           recv_string_data);
       }
     }
   }

end2:
      free(serveraddr);
end1:
      free(clientaddr);
end:
      (void)close(sockfd);

      return 0;
}

Step 6: Compile and Execute client.c

$ gcc -o client client.c

$ sudo ./client 127.0.0.1

IP Address: 127.0.0.1
IP Address: 127.0.0.1
checksum is c135
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
Received : Hello client
^C

Enhanced Socket Flexibility with AF_INET and PF_INET Domains

Default Domain:

By default, the socket is configured to work in the AF_INET domain, handling all types of network data.

Additional Domain Support:

We expand the socket’s capabilities to also function in the PF_INET domain, allowing it to operate similarly to AF_INET.

Socket Creation:

We set up a network connection point known as a socket using socket(PF_INET, SOCK_RAW, IPPROTO_TCP).

Working Scenario:

Despite the change in domain to PF_INET, the socket continues to operate the same way, handling general network data.

Summary

Socket API	Learning
socket	Create a new socket
select	Monitor multiple file descriptors (usually sockets) for read, write, or error conditions.
write	used to write data to a file descriptor, such as a socket.
read	used to read data from a file descriptor, such as a socket.

See Also

• Previous topic

  • Basic example raw af inet tcp server and client

• Current topic

  • IPV4 RAW AF INET TCP server client program with Select system call

• Next topic

  • IPV4 RAW AF INET TCP server client program with Poll system call

• Other sockets

  • ipv4_raw_af_inet_udp

  • ipv6_af_inet6_tcp

  • ipv6_af_inet6_udp

  • //TODO ipv6_raw_af_inet6_icmp

  • //TODO ipv6_raw_af_inet6_tcp

  • //TODO ipv6_raw_af_inet6_udp

  • // TODO ipv6_raw_af_inet6_raw

  • raw_af_packet_raw_htons_ETH_P_ALL

  • raw_af_packet_tcp_htons_ETH_P_ALL

  • raw_af_packet_udp_htons_ETH_P_ALL

• Other IPCs

  • Message_queues

  • NamedPipes

  • Netlink

  • Shared_Memory

  • Shared_Memory_2_FDS

  • SocketPair

  • Timerfd